Recovery of heavy oils through in-situ combustion process

ABSTRACT

An in-situ combustion process heats an oil-bearing formation so as to reduce the viscosity of heavy oil, and/or to extract oil from solid or semi-solid materials in the formation. Oxygen-enriched air for the combustion is generated non-cryogenically at the surface, preferably with a membrane system or a pressure swing adsorption (PSA) unit. The oxygen-enriched air may be blended with other air to adjust its oxygen content, and is then compressed at the surface, and conveyed into an injection well. The oxygen-enriched air is especially intended for use in a toe-to-heel in-situ combustion process, in which combustion proceeds along a horizontal well. Nitrogen resulting from the production of the oxygen-enriched air may be used to compress the oxygen-enriched air, or for other purposes.

BACKGROUND OF THE INVENTION

The present invention relates to the field of recovery of oil from oilwells, and provides an improved method for recovery of oil previouslyconsidered unrecoverable.

Oil formations typically include portions in which the oil is present inliquid form, and portions in which the oil is trapped in a solid orsemi-solid form, such as a tar, bitumen, or asphalt. Some oil exists ina heavy, viscous form, which is difficult or impossible to pump out. Theoil in the liquid form can be pumped out relatively easily, but it ismuch more difficult, and expensive, to extract the heavy oil. It isespecially difficult to extract the oil that is contained within a solidmaterial; in their natural state, materials such as bitumen will notflow at all. For these reasons, in the prior art, when the liquid in aformation has been substantially recovered, the oil well has beendeclared spent, and the remaining oil has been deemed unrecoverable.

It turns out that, for many oilfields, the amount of oil deemedunrecoverable may be as great as, or substantially greater than, theportion that is easily recoverable. Thus, if it were possible torecover, economically, all of the purportedly unrecoverable oil in knownoilfields, the amount of proven petroleum reserves available to theworld would increase by a very substantial amount.

It has been proposed, in the prior art, to recover heavy oil, and oilfound within solid materials, by heating the underground formationitself, and thereby causing heavy oil to become less viscous, and/orcausing oil trapped within a solid material to flow out of the material.In effect, heating causes formerly unrecoverable oil to become ordinary,liquid crude oil, which can be pumped out by conventional means.

An obvious problem with the concept of heating an oil reservoir is thatit is difficult to heat a formation that may be hundreds of feet, ormore, below the ground. One prior art method of providing such heat hasbeen to generate steam at the surface of a well, and to pump the steaminto the well to heat the formation.

A disadvantage of the use of steam is that the process requires a largeamount of energy, mainly through the burning of natural gas to producethe steam. Also, because steam tends to rise, it tends to flow above, oroverride, the oil reservoir into which it is injected, thereby missingmuch of the formation intended to be heated. As a result, the steamheating process may recover only about 30% of the oil in the reservoir.

The problem of steam override can be reduced by providing separatehorizontal wells, wherein the steam is injected into an upper well, andso that the resulting liquid oil flows by gravity to a production welldisposed below the first well, allowing the liquid oil to be pumped out.The latter process is an improvement over other steam heating methods,as it allows about 40-60% recovery, but it still requires burning ofnatural gas at the surface.

To overcome the problems associated with steam produced at the surface,it has been proposed to heat the formation “in-situ”, i.e. at thelocation of the formation. In theory, one can start an underground fire,combusting a small part of the formation. The fire is supported bycompressed air injected from the surface. If the compressed air is hotenough, it can ignite the formation and support combustion. Combustionof a portion of the formation generates heat which heats other portionsof the formation, thereby causing the oil in the formation to become anextractable liquid.

In practice, in-situ combustion is difficult to manage, and has achievedonly marginal results, with about 30% recovery at most. Clearly, oneseeks to burn only a small part of the formation, leaving uncombustedliquid oil to be pumped out. It is difficult to limit the scope ofin-situ combustion in this way. Also, it is important to be able tocontrol the propagation of the combustion within the formation. Onemight start a fire in a part of the reservoir, but the fire might movein random directions, based on fracture patterns in the formation. Inthe worst case, the combustion might destroy the very oil that one seeksto recover. Control of the propagation of in-situ combustion isinherently difficult.

An improved process for in-situ combustion of an oil formation isdescribed in U.S. Pat. No. 5,626,191, the disclosure of which isincorporated by reference herein. In the patented process, a verticalinjection well is positioned near a production well having horizontaland vertical portions. The production well has the general shape of afoot, and therefore defines a “toe” portion and a “heel” portion. Theinjection well provides a path for injection of air near the toe portionof the production well, and the air, and the combustion front, proceedslaterally, from the toe to the heel. This in-situ combustion process issometimes called TTH, for “toe-to-heel” combustion. More recently, theprocess has been known in the industry by the acronym THAI, meaning “toeto heel air injection”.

An improvement to the THAI process is described in U.S. Pat. No.6,412,557, the disclosure of which is also incorporated by referenceherein. The latter patent discloses a catalyst deposited in the gravelpack surrounding the production well. The catalyst, which is similar tocatalysts used in conventional refineries, not only provides bettercontrol of the combustion process, helping to prevent the entireformation from being burned, but it also chemically upgrades the oilbefore it even comes out of the ground. In particular, the catalystsupports reactions that separate undesirable substances, such as sulfur,asphaltenes, and heavy metals, from the oil. Moreover, the processinherently burns unwanted coke while the oil is still underground. Inprior art processes, the coke would have to be removed at the surface.The remnants of the burnt coke seal the horizontal portion of the well.The process including the catalyst is known in the industry asTHAI/CAPRI.

The THAI/CAPRI process has further advantages over prior art in-situcombustion methods. Entrained gases such as nitrogen rise with the oilto the surface, and can be separated from the oil and sold. Residualheat from the oil can be bled off to produce power. Water produced inthe process can be used for irrigation without additional treatment. Andthe process does not require burning of natural gas at the surface,making the process more environmentally benign. The major requirement isonly a source of compressed air, and means for forcing it into areservoir.

It is believed, based on the results of computer simulations, that theTHAI/CAPRI process could recover as much as 80% of the oil trappedwithin a reservoir, and previously deemed unrecoverable. A recoverypercentage this high has been unattainable in the prior art.

An important ingredient of the THAI/CAPRI process is compressed airwhich is injected into the formation. In the prior art, such air hasbeen derived from ambient air that has been compressed and stored in acylinder or other container. Alternatively, combustion air could besupplied by vaporizing liquid oxygen, and combining it with ambient airor nitrogen, before injecting it into the formation. But cryogenicsystems are expensive, difficult to transport, and require regularmaintenance, which can be especially difficult in remote areas.

The present invention provides an improvement of the above process, byproviding a more desirable means of generating an oxygen-rich gas forsupporting in-situ combustion.

SUMMARY OF THE INVENTION

The present invention comprises an improved in-situ combustion processfor recovery of oil from a formation. The in-situ combustion processgenerates heat which causes heavy oil in a reservoir to become lessviscous, and thus to flow readily to a location from which it can bepumped out. In-situ combustion is also used in order to release oil thatis trapped in a solid or semi-solid material, such as bitumen.

An in-situ combustion process requires that air be supplied to thelocation of the combustion. According to the present invention, thecombustion air is oxygen-enriched air that is generatednon-cryogenically above the surface and injected into a well. Theoxygen-enriched air may be produced by a membrane system or a pressureswing adsorption (PSA) system. Preferably, the oxygen-enriched air iscompressed before being injected into the well.

The non-cryogenically produced oxygen-enriched air may be blended withambient air, or other air, so as to produce a gas having a desiredoxygen content. In this way, the oxygen content of the gas can be easilyadjusted to suit the requirements of the particular application.

The membrane or PSA system produces an oxygen-enriched stream and anoxygen-depleted stream. In another embodiment, the oxygen-depletedstream, which is normally mostly nitrogen, can be used to operate acompressor for compressing the oxygen-enriched stream before it isinjected into the well. The nitrogen could be used instead for otherpurposes.

The present invention is especially useful with a toe-to-heel combustionprocess, which is a process in which air is conveyed into an injectionwell, to support combustion in a horizontal well having an identifiable“toe” and “heel” structure. The present invention providesoxygen-enriched air in the latter process, instead of ordinary air. Theinvention is also useful in an improved version of the toe-to-heelcombustion process, in which the process employs a catalyst whichchemically upgrades the oil before the oil is pumped out of the ground.

The present invention therefore has the primary object of providing animproved process for extracting heavy oil, and/or oil trapped in a solidor semi-solid material, from an underground formation.

The invention has the further object of providing an economical andconvenient source of oxygen-enriched air, for use in an in-situcombustion process for oil recovery.

The invention has the further object of reducing or eliminatingenvironmental hazards associated with an in-situ combustion process foroil recovery.

The invention has the further object of providing an improved processfor oil recovery, wherein the process can be operated with equipmentthat is readily portable.

The invention has the further object of providing oxygen-enriched airfor use in an in-situ combustion process for oil recovery, wherein theoxygen content of the oxygen-enriched air can be easily adjusted to suitthe requirements of the combustion process.

The reader skilled in the art will recognize other objects andadvantages of the present invention, from a reading of the followingbrief description of the drawing, the detailed description of theinvention, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWING

The Figure provides a schematic diagram of an apparatus used forpracticing the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention includes a method and apparatus for providingoxygen-enriched air for use in recovering heavy oil, or oil that istrapped in a solid or semi-solid material, from an underground oilreservoir. The oxygen-enriched air is used in an in-situ combustionprocess, wherein the air is injected into a well to support combustionof a portion of the reservoir, so as to generate heat in the formation.The resulting heat causes the uncombusted oil in the vicinity of thecombustion to become less viscous, so that the oil can flow to alocation from which it can be removed by conventional means.

In particular, the present invention can be considered an improvement tothe THAI, or THAI/CAPRI process, for oil recovery, described above. Moregenerally, the invention can be used with any in-situ combustionprocess, wherein a portion of an oil formation is combusted to heat anadjacent portion of the same formation. The invention is therefore notnecessarily limited to use with THAI or THAI/CAPRI processes.

In its basic form, the invention comprises generating oxygen-enrichedair, using a non-cryogenic process, above the surface of a well, andinjecting the oxygen-enriched air downhole to support combustionunderground. The non-cryogenic process preferably uses a gas separationmembrane system or a pressure swing adsorption (PSA) system. The oxygencontent of the oxygen-enriched air may be adjusted by mixing theoxygen-enriched air with ambient air, or other air, to obtain a streamhaving a desired percentage of oxygen. Controlling the percentage ofoxygen provides additional control over the downhole combustion, andenables the operator to tailor the composition of the air to theparameters of the oilfield, including the type of oil to be recovered,the temperature of the reservoir, etc.

In another embodiment, the invention includes capturing theoxygen-depleted gas stream produced by the membrane or PSA system, andusing this stream to drive a turbocompressor that further compresses theproduct oxygen-enriched gas.

The use of oxygen-enriched air, instead of ordinary air, in supportingin-situ combustion, has several advantages. Oxygen-enriched air reducesthe amount of gas needed for combustion, because of its higher oxygencontent. Because different reservoir sites have different parameters(i.e. type of oil, temperature, etc.), the same system can be used atdifferent sites, possibly with different settings of the oxygen contentof the injected air. Thus, the present invention effectively increasesthe number of viable reservoir sites from which oil can be recoveredwith the same system. It is easy to adjust the oxygen content, simply bymixing the product oxygen-enriched gas with ambient air.

The use of oxygen-enriched air produced by a membrane or a PSA systemhas advantages over the use of a cryogenic system, in that membranes orPSA systems cost less than cryogenic systems, are more readily portable,and are easier to use. Membrane or PSA systems do not require costlyequipment for storage and transport of cryogenic liquids.

The Figure provides a schematic diagram of the components of a systemused to practice the present invention. Air compressor 1 takes ambientair and compresses it. The compressed air passes through receiver 2 andmoisture separator 3. The air may then pass through an optional airdryer 4. The air passes through coalescing filters 5 and 6, and heater7. Air leaving the heater may pass through an optional carbon bed 8, andthen through particulate filter 9.

Air leaving the particulate filter enters air separator 10, which may beeither a membrane or a pressure swing adsorption (PSA) unit. The airseparator converts the incoming air into two streams, one which isoxygen-enriched and the other which is oxygen-depleted. In the extremecase, the separator may produce one stream that is virtually all, ornearly all, oxygen, and another stream which is almost all nitrogen. Inthe more general case, the separation of oxygen is less than complete.

The oxygen content of the output of the separator 10 may be controlledby blending air, which may be ambient air or other air, with theoxygen-enriched stream as it exits the separator. The Figure showsblending air being injected through conduit 13. Conduit 13 is preferablypositioned upstream of all compressors in the system, so that the airstreams may be blended at ambient pressure.

The resulting product stream comprising oxygen or oxygen-enriched air iscompressed in compressor 12. The output of compressor 12 is a compressedoxygen-enriched gas stream, which is then directed to an injection well,for supporting an in-situ combustion process, as discussed above.

In an alternative embodiment, also illustrated in the Figure, thenitrogen (or, more generally, the oxygen-depleted gas stream produced bythe membrane or PSA system) is used to drive a turbocompressor 11, whichhelps to boost the pressure of the oxygen-enriched product gas. Thelatter arrangement is especially useful in situations where the nitrogen(or oxygen-depleted air) is not needed, or not needed at pressure, sincethe normal operation of a membrane would yield nitrogen at near feedpressure of the membrane system.

Alternatively, the nitrogen (or oxygen-depleted gas) produced by theseparator 10 can be used for inerting the oil product, or for any otheruse in which nitrogen or an oxygen-depleted gas is needed in thevicinity of an oil well. Such nitrogen could be used for enhancing oilrecovery, for drilling, or for other uses.

The nitrogen or oxygen-depleted air produced by the separator could bediscarded instead of being used as stated above. The present inventionis intended to include this possibility also.

The method described above has significant advantages over the priorart. The components shown in the Figure can be provided in a housingwhich can be relatively easily moved from one oil recovery site toanother. The use of oxygen-enriched air to support in-situ combustioncan reduce the amount of gas needed to support such combustion, and caneffectively increase the number of viable reservoir sites from which oilcan be recovered.

Membrane systems can provide up to about 60% oxygen, while PSA systemscan provide up to about 99% oxygen, using currently-availabletechnology. Membrane systems may be preferred over PSA systems, in thatby limiting the oxygen content to 60%, it may be possible to avoidsafety precautions that would be required for gases having a higheroxygen content.

The invention can be modified in various ways. Any or all of theelements in the Figure labeled “optional” can be omitted or included.The oxygen-enriched air can be used to support various kinds of in-situcombustion processes, not just the THAI or THAI/CAPRI processes. Thesemodifications, and others which will be apparent to those skilled in theart, should be considered within the spirit and scope of the followingclaims.

1. In an in-situ combustion process for oil recovery from a formationlocated below a surface, the combustion process including combusting aportion of an oil-bearing formation so as to generate heat in theformation in order to reduce viscosity of heavy oil in the reservoirand/or to cause oil trapped in a solid or semi-solid material to bereleased, in liquid form, from the material, the improvement comprisingnon-cryogenically generating an oxygen-enriched gas stream above thesurface and injecting said oxygen-enriched gas stream into the formationso as to support in-situ combustion in the formation.
 2. The improvementof claim 1, wherein the non-cryogenic generating step includes conveyingair through a membrane system.
 3. The improvement of claim 1, whereinthe non-cryogenic generating step includes conveying air through apressure swing adsorption system.
 4. The improvement of claim 2, furthercomprising the step of blending air with the oxygen-enriched gas streamso as to produce a gas stream having a desired oxygen content.
 5. Theimprovement of claim 3, further comprising the step of blending air withthe oxygen-enriched gas stream so as to produce a gas stream having adesired oxygen content.
 6. The improvement of claim 1, wherein thegenerating step produces an oxygen-enriched stream and anoxygen-depleted stream, the process further comprising using theoxygen-depleted stream to operate a compressor for compressing theoxygen-enriched stream.
 7. The improvement of claim 1, wherein thein-situ combustion process is selected to be a toe-to-heel combustionprocess.
 8. The improvement of claim 7, wherein the in-situ combustionprocess is selected to be a process which uses a catalyst to upgrade oilbefore the oil has been withdrawn from the formation.
 9. The improvementof claim 1, further comprising compressing the oxygen-enriched gasstream before injecting said stream into the formation.
 10. A method ofenhancing an amount of oil recoverable from a formation, the formationbeing located beneath a surface, the method comprising: a)non-cryogenically generating an oxygen-enriched gas stream above thesurface of the formation, and b) injecting said oxygen-enriched gasstream into the formation so as to support combustion of a portion ofthe formation, wherein the formation is heated by said combustion so asto reduce viscosity of heavy oil in the reservoir and/or to cause oiltrapped in a solid or semi-solid material to be released, in liquidform, from the material.
 11. The method of claim 10, wherein thenon-cryogenic generating step includes conveying air through a membranesystem.
 12. The method of claim 10, wherein the non-cryogenic generatingstep includes conveying air through a pressure swing adsorption system.13. The method of claim 11, further comprising the step of blending airwith the oxygen-enriched gas stream so as to produce a gas stream havinga desired oxygen content.
 14. The method of claim 12, further comprisingthe step of blending air with the oxygen-enriched gas stream so as toproduce a gas stream having a desired oxygen content.
 15. The method ofclaim 10, wherein the generating step produces an oxygen-enriched streamand an oxygen-depleted stream, the process further comprising using theoxygen-depleted stream to operate a compressor for compressing theoxygen-enriched stream.
 16. The method of claim 10, wherein the in-situcombustion process is selected to be a toe-to-heel combustion process.17. The method of claim 16, wherein the in-situ combustion process isselected to be a process which uses a catalyst to upgrade oil before theoil has been withdrawn from the formation.
 18. The method of claim 10,further comprising compressing the oxygen-enriched gas stream beforeinjecting said stream into the formation.
 19. A method of recovering oilfrom an oil-bearing formation, comprising: a) selecting a location ofthe formation, and determining parameters relating to the formation, b)choosing a level of oxygen content for air to be injected downhole insaid formation to support in-situ combustion, the oxygen content beingchosen in accordance with said parameters, c) generating anoxygen-enriched gas above a surface of said formation, and adjusting anoxygen concentration of said oxygen-enriched gas according to the levelchosen in step (b), and d) injecting said oxygen-enriched gas to acombustion site below the surface of the formation.
 20. The method ofclaim 19, wherein step (c) is performed by a portable system, andwherein method further comprises the steps of repeating steps (a) and(b) for a different formation, moving the portable system to a vicinityof said different formation, and repeating steps (c) and (d) in thevicinity of said different formation.